He was also heavily invested in sport — he owned the NBA's Portland Trail Blazers, the NFL's Seattle Seahawks, and had a stake in the Seattle Sounders soccer team

Mr Gates, who founded Microsoft Corp in 1975 with Mr Allen, released a statement following his friend's death.

"I am heartbroken by the passing of one of my oldest and dearest friends, Paul Allen," Mr Gates said.

Mr Allen was two years older than Mr Gates, but when they met in the computer room at the exclusive Lakeside School in Seattle in 1968, they discovered a shared passion.

"In those days we were just goofing around, or so we thought," Mr Gates recalled in his 1985 book The Road Ahead.

Microsoft's big break came in 1980, when IBM Corp decided to move into personal computers and asked Microsoft to provide the operating system — a decision that would make the two Seattle natives billionaires.

They did not invent the operating system; to meet IBM's needs, they spent $US50,000 to buy one known as QDOS from another programmer, Tim Paterson.

Eventually the product refined by Microsoft — and renamed DOS, for Disk Operating System — became the core of IBM PCs and their clones, catapulting Microsoft into its dominant position in the PC industry.

The first versions of two classic Microsoft products, Microsoft Word and the Windows operating system, were released in 1983.

By 1991, Microsoft's operating systems were used by 93 percent of the world's personal computers.

Visionary and philanthropist

Over the course of several decades, Mr Allen gave more than $US2 billion to a wide range of interests, including ocean health, homelessness and advancing scientific research.

Mr Allen, the owner of 42 US patents, liked to cast himself as a technology visionary who drove Microsoft's early success and saw the future of connected computing long before the internet.

"I expect the personal computer to become the kind of thing that people carry with them, a companion that takes notes, does accounting, gives reminders, handles a thousand personal tasks," he wrote in a column in Personal Computing magazine as far back as 1977, long before portable computers became a reality.

In the same year, he outlined an early vision of what turned out to be the internet to Microcomputer Interface magazine.

"What I do see is a home terminal that's connected to a centralised network by phone lines, fibre optics or some other communication system," he said.

"With that system you can perhaps put your car up for sale or look for a house in a different city or check out the price of asparagus at the nearest grocery market or check the price of a stock."

Mr Allen later called this sweeping idea the "wired world," which has broadly come to fruition. He was not alone in predicting connected computing, but was one of the most prominent.

Yet his technology ventures after Microsoft, which focused on areas he thought would grow with the advent of the "wired world," were not as successful.

He lost $US8 billion in the cable television industry, chiefly with a bad bet on cable company Charter Communications, while technology ventures he bankrolled such as Metricom, SkyPix and Interval Research were costly failures.

Sports teams, real estate and rock 'n' roll

Mr Allen had better luck in sports and real estate, buying the Portland Trail Blazers basketball team in 1988 and becoming a local hero in 1997 when he purchased the Seattle Seahawks football franchise after the previous owner had tried to move the team to California.

The Seahawks won the Super Bowl in February 2014 and both franchises are now valued at many times what he paid for them.

Never married, Mr Allen also made hundreds of millions of dollars redeveloping a shabby area of downtown Seattle that became a gleaming technology Mecca and the site of Amazon.com's glass "spheres" headquarters.

All the while he pursued numerous personal projects and pastimes. He owned one of the world's biggest yachts, the 122-metre Octopus, which was the venue for many lavish parties and the base for scuba expeditions.

A rock 'n' roll aficionado, Mr Allen had a band on call to jam with when he wanted, and spent more than $US250 million building a museum devoted to his hero, Jimi Hendrix, which morphed into a music and science fiction exhibit designed by Frank Gehry.

He spent millions more on a collection of vintage warplanes and funded the first non-government rocket to make it into space.

He also collected priceless antiquities and works by Monet, Rodin and Rothko to put in his extensive art collection.